SAN FRANCISCO — Google plans to include a dozen or so massive sails on the four-storey barge under construction in the heart of the San Francisco Bay, creating a floating artistic structure the Internet giant promises will “stand out.”

After weeks of speculation that the barge would be “a party boat,” a data-storage centre and a store to sell its Internet-connected glasses, Google revealed it plans to use the vessel as an “interactive space where people can learn about new technology.”

The San Francisco Chronicle reported Friday it got planning documents for the vessel from the Port of San Francisco through a public records request.

The documents shed more light on a project shrouded in mystery. Google has been tight-lipped about the barge and has managed to conceal much of its purpose by constructing it on docked barges, instead of on land, where city building permits and public plans are mandatory.

The newspaper said the sails shaped like fish fins are meant to instil in visitors a sense of seaworthiness while aboard the boxy 15-metre-tall, 76-metre-long structure made of recycled shipping containers. The documents submitted to the port boast the completed project will be an “unprecedented artistic structure” that will “stand out.”

A company called By and Large LLC submitted the planning documents. It has close ties to Google and some are speculating the name is a play on the word “barge.” Records and other official accounts identify the project as Google’s.

The documents refer to the barge as a “studio” and “temporary technology exhibit space” that will “drive visitation to the waterfront.”

The Internet giant’s actions at Treasure Island appear legal. But the mystery surrounding the bulky floating building — and a similar one off Portland, Me. — is generating rumours and worries.

Google has dodged public scrutiny by essentially constructing a boat, not a building. Thus it does not need permits from San Francisco, a city with copious inspection and paperwork requirements for builders.

Still, privacy experts, environmentalists and legal authorities say the company’s refusal to divulge many details and the air of secrecy it has created around the project may backfire because Northern California residents are highly protective of one of the most scenic and environmentally sensitive bays in the U.S.

Environmental groups and others have hinted at filing lawsuits if they feel the final product will harm the bay.

The planning documents say there are plans to sail the boat around the bay, mooring at different docks monthly. Google also has ambitions to sail the vessel out of the bay and up and down the West Coast.

The planning documents were submitted in September. San Francisco Port spokeswoman Renee Dunn Martin said the documents are “part of a preliminary proposal. They haven’t come back to us with anything concrete.”